---
tags:
  - C
---

Think of them as a find and replace that takes place before compilation.

They are ephemeral and never make it into the compiled code.

With a macro, the computation happens "inline" without having to call functions
which make costly alterations to the stack.

A function and function call such as:

```c
int square(int x) { return x * x; }
int a = square(5);
```

Can be reduced to a macro:

```c
#define SQUARE(x) ((x) * (x))
int a = SQUARE(5);
```

Which, because it uses `#` is processed at the
[pre-processor compilation stage](./C_compilation_process.md), becomes:

```c
int a = ((5) * (5));
```

after pre-processing but before compilation.

## Syntax

`#define` always creates a macro but there are different types.